This invention relates generally to reflective optical materials, and more particularly to reflective diffraction and interference-type optical elements, such as optical filters and combiners, which are used, for example, in head-up displays or helmet-mounted visor displays.
In various optical systems, it is often necessary to provide a filter in order to remove undesired radiation while at the same time allowing desired radiation to be efficiently transmitted or reflected. Such filters and coatings are used, for example, to provide protection from laser radiation for personnel, for electro-optical detectors, and for optical mirrors in a laser system, as a holographic lens in a head-up display system, or in night vision devices The optical filters currently used for such purposes include absorption filters, reflective multiple layer dielectric filters, and diffraction filters generated by optical holographic techniques. However, each of these approaches to providing optical filters has certain disadvantages, as discussed below.
The absorption filter comprises a material which is impregnated with absorption dyes or materials with intrinsic absorption at the wavelength of the incoming laser radiation, as described, for example, in the book entitled "Handbook of Optics", W. G. Driscoll, ed., McGraw-Hill Book Co., New York, 1978, in Section 8 (Coatings and Filters), at pages 7 to 32. This type of protection has the serious disadvantage that the absorbing dye decreases the amount of transmitted radiation to unacceptably low levels. In addition, for laser applications, as the laser radiation energy increases, the radiation can damage the protective filter itself.
The reflective multiple layer dielectric filters typically consist of alternate layers of two dielectric materials of different refractive indices, which are formed on the surface of a substrate by known deposition techniques, such as chemical vapor deposition, sputtering, or thermal evaporation. When the optical thickness of each layer is chosen to be one-quarter of the wavelength of the radiation being reflected, such a structure is referred to as a "quarterwave stack", as discussed, for example, in U.S. Pat. No. 4,309,075 and in the book entitled "Handbook of Optics", previously referenced, in particular in Section 8. However, there are limitations on the spectral bandwidths which can be achieved by such structures, because of the limited material combinations available and the resulting restriction on the choices of index modulations. Moreover, defects at the abrupt interfaces between the layers in a multilayer structure can cause unwanted optical scattering. In addition, these defects can cause excessive absorption of radiation by the dielectric material, which can result in thermal damage to the optical filter. Furthermore, in a multilayer dielectric coating, the electric field is strongest at the interface regions between the high index material and the low index material. This highly localized field occurring at the abrupt interfaces can produce maximum temperature increases. Since the thermal expansion coefficients are different for the different dielectric materials of adjacent layers, high thermal stress is developed at the interface regions, which could cause delamination of the successive layers in the film. In addition, the high thermal stress could create microscopic dislocations which result in unwanted optical scattering by the film. Further, substrate roughness, pinholes and contaminants in the conventional multilayer structures formed by evaporation or sputtering techniques increase absorption and scattering, generate localized heating, reduce maximum reflectivity, and increase radiation damage. Finally, these multilayer coatings exhibit reflectance peaks at multiple wavelengths, which causes reduced optical transmission.
Diffraction optical elements have been generated using known methods of optical holography in photosensitive gelatin material, as discussed, for example, in the book entitled "Optical Holography" by Collier, Burckhardt, and Lin, Academic Press, New York, 1971, Chapter 9 (Diffraction from Volume Holograms) and Chapter 10 (Hologram Recording Materials), as well as in the book entitled "Handbook of Optical Holography", by Caulfield, Academic Press, New York, 1979, Chapter 10 (Application Areas). However, gelatin diffraction elements have environmental stability problems and are susceptible to degradation by humidity and heat. In order to overcome this problem, a protective layer such as glass or a glass-like coating can be used, but such a layer complicates the manufacturing process and adds to unit cost. Moreover, such gelatin filters are limited to use for radiation in the wavelength range from the visible to the near infrared (i.e., up to about 2 microns) since sensitized gelatin is not sensitive to longer wavelength exposures. Consequently, filters for infrared applications cannot be fabricated in a gelatin structure. In addition, the index modulation in the gelatin, which is produced by exposure to the holographic interference pattern and subsequent development, is limited to a shape approximating a sinusoidal configuration or a roughly superimposed multiple sinusoidal configuration. Furthermore, the fabrication of a gelatin filter requires numerous steps, in particular numerous wet chemical steps for development, which are sensitive to processing variables, such as temperature or vibration, that affect the efficiency and peak wavelength of the final structure. In addition, since the resistance of gelatin to damage by heat or radiation is relatively low, gelatin filters are limited to low power applications. Finally, fabrication of a filter which reflects radiation at two selected wavelengths requires multiple exposure of the gelatin to two holographic patterns, which produces an irregular index profile that reduces the efficiency of the filter.
One general application in which gelatin filters have heretofore been employed is that of the optical combiner element of a reflective display, such as a head-up display (HUD) or helmet visor display (HVD) commonly used in aircraft display systems. U.S. Pat. No. 3,940,204 discloses exemplary HUD and HVD systems. The laminated gelatin holographic combiner employed for these applications typically comprises a spherical plastic substrate to which are bonded successive layers of glass, the gelatin hologram, glass, plastic and an antireflective (AR) coating. The glass layers sandwiching the gelatin are required to protect the gelatin from degradation by humidity. As a result of the multiple layers, strong undesirable ghost images may be produced by the gelatin holographic combiners.
Combiners for display systems can be designed to compensate or balance aberrations in the display system. The compensation may comprise the implementation of aspheric reflective layers or surfaces. With the state of the current technology it is not economically feasible, on a production basis, to provide glass layers or substrates with aspheric surfaces. Instead the required asphericity is incorporated into the gelatin hologram itself, which means that the fringes will be slanted varying degrees with respect to the gelatin surfaces. This creates a grating at the hologram surface and results in a phenomenon known as chromatic dispersion, wherein the direction of light diffracted from the hologram is wavelength dependent. In a holographic display such as the HUD or HVD, if the display light source has any appreciable spectral bandwidth, chromatic dispersion will blur the image at the exit pupil, perhaps to an unacceptable level. Even with narrow band light sources, such as a cathode ray tube (CRT) with P43 phosphor, the fringe slant in some areas of the hologram may be large enough to cause significant dispersion-induced degradation of the image. Slant fringes may also result in flare, a condition in which extraneous diffraction images are produced. The extraneous diffraction may obscure the field of view.
A gelatin holographic combiner for a HUD or visor display is relatively complex and expensive to fabricate. For example, a typical gelatin holographic visor having impact resistance consists of a multi-layer laminant in which the gelatin hologram is sandwiched between two pieces of glass for humidity protection and then laminated between two pieces of polycarbonate visor for impact requirements. Antireflective coatings are applied to the respective outer surfaces of the polycarbonate pieces. The multiple laminate adds weight and complexity to the system. The gelatin holographic HUD combiner is similarly complex and heavy.
The weight of the combiner is an important consideration in the weight-critical cockpit environment. As a result of the relatively high weight of the visor gelatin hologram combiner in an HVD, the visor display center of mass is moved away from the pivot point of the pilot's head, so that the burden on his neck is increased. The increased cantilevered mass in the HUD gelatin hologram combiner decreases the combiner stiffness and resistance to vibration.